Vertebrates consist, among others, of the Aves classe containing all the birds. Birds are bipedal, warm-blooded, feathered and in most occasions flying animals. There are indeed some species -both extant as well as extinct, that are only able to walk or run. Birds are warm-blooded and lay eggs. Most likely, birds originate of a theropod group within the Dinosauria. The first remnants of birds or birdlike creatures have been found in Jurassicsediments. Some feathers in those early birds are asymmetrical, which leads scientists to conclude that the ability of flight was already well established at that time. Recent literature also suggests that many different Dinosaur groups were feathered as well. The ability to fly, however, seems to be exclusive to birds. Contrary to non-birdlike dinosaurs, birds did survive the mass extinction event at the KT boundary.

One of the most iconic birdfossils: Archaeopteryx lithographica from the Bavarian Jurassic, Germany (Foto H. Raab)

Shenzhouraptor, litteraly an early bird, found in Cretaceous strata, China (reconstruction by Matt Martyniuk)

Fossilized bird bones can be recognized by the fact that they're completely hollow or contain large air pockets.